Date of Award

12-1997

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Environmental Science

First Advisor

Abdulrahman S. Al Sharhan

Second Advisor

Mahdy M . Abdul Rahman

Third Advisor

Dr. Zeinelabidin S. Rizk

Abstract

Groundwater from Wadi AI Bih is still a major source of water required for irrigation, industry and domestic purposes in Ras AI Khaimah area, United Arab Emirates. Because of the continuous population growth, demand for pumping more water has created an imbalance in the groundwater resources of Wadi AI Bih basin, and the estimated discharge (11 million m3/year) has exceeded the calculated natural recharge (9 million m3/yr). To restore the balance between natural recharge and discharge and maintain a good-quality groundwater, the UAE government has built Wadi AI Bih dams in 1982.

The objectives of this study are to use the time-domain electromagnetic survey (TDEM), geophysical borehole logging, and hydrochemical measure-ments to investigate the subsurface conditions of Wadi AI Bih limestone aquifer, evaluate the aquifer's hydrochemical characters, and assess the impact of the dam on groundwater recharge.

Results of the TDEM soundings revealed the presence of four layers. Layer 1 is from 15 to 20 metres in thickness over the project area and has an average resistivity values of about 104 ohm.m, indicating a good aquifer. However, this layer is above the regional groundwater table and is unsaturated. The low resistivity areas are probably due to an increased clay content that could impede infiltration. This layer can be interpreted as loose superficial wadi gravels. The base of layer 2 is 70 to 100 m below ground level, and its average thickness is 63 m. This layer shows remarkable variations in resistivity (15 to 475 ohm.m), and is interpreted as cemented wadi gravels. The low resistivities probably correspond to clay rich areas. The lowermost portion of layer 2 may be below the groundwater table. i.e. within the saturated zone. As the recharge water from wadi floods has to percolate through this layer, the clay rich, low resistivity areas as well as the very resistive, highly cemented sediments may reduce local infiltration rates. The average thickness of layer 3 ranges from 5 to 95 m. The shallow areas show a good correlation with surface topography, indicating that the base of this layer defines the subsurface limestone topography. The resistivities of layer 3 are generally low ranging from .7 to 60 ohm.m and representing a transition zone between the base of the alluvial gravels and the top of the weathered limestone bedrock. Layer 3 is the main groundwater producing horizon in Wadi Al Bih area and its thickest areas should yield the highest amount of groundwater. The resistivity of layer 4 is generally above 500 ohm.m. This layer can be interpreted as an extremely resistive limestone.

Geophysical borehole logging revealed that the upper unit consists of limestone gravels (70%) and boulders (30%). The caliper log also shows that the borehole wall is uneven, confirming the gravel/boulder lithology. The second layer consists of gravels cemented with about 30% clays indicated by peaks on the gamma ray log. A slightly higher resistivity ophiolitic gravels occur between the base of the cemented gravel and the top of the dolomitic limestone bedrock. This dolomitic limestone has resistivities reaching more than 2500 ohm.m in some wells. The water level at the time of logging was either within the lower part of the gravel or in the underlying dolomitic limestone layer. Porosities are low with density porosity (φD) values mostly in the range 2-10% limestone porosity, whereas the neutron porosity (φN) values are higher. The calculated true porosity of the saturated zones varies between 8% and 25%. Two fracture zones at depths of about 100 and 150m in the penetrated formations are interpreted from the caliper-log data and indicate the presence of two distinct groundwater flow systems of different salinities and water types. Groundwater movement in the fracture zones were shown by the temperature-gradient logs, and water quality was evaluated from the fluid conductivity logs.

In September 1996, the hydraulic head in Wadi Al Bih aquifer within the study area varied from 27 m above mean sea level (amsl) on the upstream side of Wadi Al Bih dams and 19m amsl in the Ministry of Electricity and Water new well field, showing a cone of depression. The groundwater salinity and concentration of ions, except HCO3 -, show a steady increase from east to west in the direction of groundwater flow. Concentrations of chromium, cobalt, zinc, manganese, cadmium, nickel and strontium in Wadi Al Bih groundwater are below the WHO and GCC standards for drinking water, whereas the concentrations on iron, lead are slightly above these standards. The calculated groundwater-dissolved salts change from Ca(HCO3) in the upstream to CaSO4 and MgSO4 in the central area to NaCl in the west. This shows the influence of groundwater recharge in the east and the effect of saline-water intrusion in the west. The NaCl ratio indicates a brine upcoming under the MEW well field and salt water intrusion from the Arabian Gulf in the west. The calculated SAR indicated a little sodium hazard in the eastern part and medium sodium hazard in the western pm1 if Wadi Al Bih groundwater were used for irrigation.

Evidences of groundwater recharge were revealed by the seasonal variations in groundwater temperature and salinity, fluid-temperature logs and fluid-conductivity logs. The geologic structures and karstification have the main control on groundwater recharge to Wadi AI Bih area.

A comprehensive study is needed in order to achieve sustainable development of Wadi Al Bih aquifer and maintain safe yield.

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